I don't think this questionnaire is appropriate, why do you need to know people's medical problems if you want to find out their opinion on GM organisms/crops.

Some researchers have done experiments on laboratory animals feeding them solely GM crops; the animals seemed to have GI tract and cardiac symptoms. Of course, there would be other people arguing this was flawed because the laboratory animals lack other nutritions.

Because I propose to do a comparison studies between people eating mainly GM crops and those who don't eat the crops as often, I thought of doing a survey or questionnaire based on the results of previous studies...

I'm sorry, but this is not how you do research. This is why there are so many useless association studies that contradict each other in their conclusions. In order to do a proper study you at least need to take into consideration all the known variables that could lead to GI tract/cardiac symptoms, you also at least need to accurately document the variable that you are trying to evaluate an association of. You simply cannot make a trivial questionnaire and think you will get reliable data out of it.